Cytological studies of growing cells in the root

Part A. Onion root tips were grown in water or cytokinin. Levels of RNA in the chromatin region of nuclei were analyzed using visible light microscopy with basic-dye staining, and ultraviolet microscopy of unstained material. No evidence was found for a significant increase in nuclear RNA in response to cytokinin treatment.Part B. The apical ten millimetres (segments 1 to 10 from the apex) of pea-seedling roots were compared cytologically.The apex contained smooth nuclei and large, granular nucleoli; chromatin was moderately condensed. Ribosomes were mainly unbound and there was little endoplasmic reticulum. During elongation (segments 3-7) nuclei enlarged and became lobed; nucleoli grew smaller and largely fibrillar. Chromatin in some nuclei of segments 5-9 was very condensed. Chromatin was most dispersed in segment 3; a correlation between this structure and DNA synthesis is suggested. During elongation, the amount of rough endoplasmic reticulum (RER), the density of ribosomes on membranes and polysome length increased, as did the electron-density of the background matrix. The number of unbound ribosomes decreased.A decrease in the intensity of RNA synthesis was observed autoradiographically and is consistent with the ultrastructural changes described.Part C. Mitochondrial crystals in pea-root epidermal cells were examined in the electron microscope. Crystals in sections were usually rectangular and were located in the mitochondrial matrix. Treatment of sections with pronase removed the crystals. In a negatively-stained mitochondrial fraction, crystals were observed in mitochondria and free; they were always rectangular, but varied in thickness.Three repeating planes formed five types of crystalline lattices; distances between repeating planes were calculated from optical diffraction patterns. A model suggesting a possible disposition of subunits within the crystalline lattice is presented. The function of the crystals and their relationship to cell development are discussed also.